(1) Field of the Invention
The invention generally relates to direct current machines. More particularly, the invention relates to an electrically or permanent excited brushless direct current machines. Disclosed embodiments may be used as a motor for propelling a bicycle or other vehicle, as a generator for energy recuperation and as a battery charger.
(2) Description of the Related Art
German patent DE 199 83 946 B4 discloses a system or machine of two components, sometimes called a rotor and stator, wherein the rotor and stator are concentrically orientated to one another. One of the two components, typically either the rotor or the stator, has a specific number of magnets, such as permanent magnets, wherein the magnets are alternatively magnetized north and south, and the respective other part (either the rotor or stator) has a specific number of coils which are formed by teeth around which insulated wire is wound. Between these coils there are formed respective slots. Accordingly, these slots are the gaps between adjacent teeth. Between these coils there are formed respective slots.
The concept whether the rotor is disposed within or outside the stator and which of both components is provided with the permanent magnets, while then the other component has the coils and slots, is particularly determined or suggested by the respective use or application of the machine. Document DE 199 83 946 B4 discloses a permanent magnet brushless direct current machine which is used as a drive for a bicycle in a wheel in the form of a hub motor, wherein the central part is disposed in a stationary manner and therefore is the stator and has the coils and the slots, respectively, and where the rotor is an outer rotor provided with permanent magnets. Fed by a rechargeable battery as a power source, the motor can be used as a drive as well as a generator for energy recuperation and charging of the battery. The drive of the machine is effected in a normal manner via a current controlled inverter having three phases.
The number of poles, i.e. magnetic poles, and the number of slots, i.e. coils, are significant features in the design of such a machine. Document DE 1 99 83 946 B4 discloses for instance a kind of machine having twenty-two poles and twenty-four slots and a further machine variant having forty-four poles and forty-eight slots.
In addition to the numbers of poles and slots, the so called pole-slot-combination, there are further features of such a machine which are decisive for its characteristics. These include the kind of winding and the manner, in which the winding of the coils is connected, i.e. the so called coil groups wiring concept. The coils can be combined in groups and the wiring can be in the form of a star connection or in the form of a delta connection. Document DE 1 99 83 946 B4 for instance discloses the winding pattern C′, C, C′, C; B, B′, B, B′; A′, A, A′, A; C, C′, C, C′; B′, B, B′, B; A, A′, A, A′ in case of an assembly having two coils per slot and an assembly having four coils in one group.
The kind of winding and the connection of the windings, i.e. the wiring concept as well as the assembly in groups, have to be taken into account already upon design of a machine. Respective terminals have to be planned and must be provided and a wiring machine which requires a substantial investment volume has to be designed and programmed.
In case of the winding pattern according to DE 1 99 83 946 B4, specifically as shown in FIG. 4 of this document, winding of each tooth in a group of four teeth starts with a winding in the counter clockwise direction (C′), followed by a winding in a clockwise direction (C), wherein the first group of four ends with a clockwise winding direction (C′CC′C) and the next group of four for the next phase, namely phase C in the present case, starts with a winding in clockwise direction C and ends in counter clockwise direction C′. For the sake of clarity, intervening coil groups, such as B and A are not shown between the two C coil groups of FIG. 4.
This known winding pattern as shown in FIG. 4 of DE 1 99 83 946 B4 is shown, for the sake of clarity, in a plan view of a corresponding stator in FIG. 3 of the present application. The known winding pattern and the respective winding directions are also shown, in an enlarged and in a sectional view, in FIG. 4 of the present application. As can be taken from the depictions of FIG. 4, the first coil group of a phase, such as phase U having four coils, seen from the left-hand side of the depiction, starts with a counter clockwise (ccw) winding (as depicted by a curved arrow above the coil) for the first coil and the coil group ends with a clockwise winding (cw) for the fourth coil, seen from the left-hand side. The fifth coil from the left-hand side, that is the first coil of the second phase U coil group, four coils, is wound in a clockwise direction and the eighth coil seen from the left hand side, that is the fourth and last coil of the second phase U coil group having four coils, is wound in a counter clockwise direction. Thus, in the prior art, coil groups of the same phase have different coil configurations.
Accordingly, this known winding pattern requires that there must be provided and wound two different coil groups of four coils, namely one coil group of four coils where the winding starts in a clockwise direction and another coil group of four coils where the winding starts with a counter clockwise direction. This is detrimental for a number of reasons as explained in the following.
Usually, stator cores of electric motors of the present kind consist of a package of single metal sheets being superimposed. A typical stator core has for instance 40 to 50 layers of single metal sheets. The single metal sheets are usually punched from metal sheet strips which are wound from coils. The punching can be effected either by punching complete rings or by punching segments of four. As shown in FIGS. 5 and 6, punching of segments of four leads to less waste then punching of complete rings. Furthermore, the punching device for a segment of four can be smaller and therefore less expensive.
In the course of manufacturing of an electric motor, the stator core is covered with insulation. Such stator insulation for instance can be done by injection molding. In order to do so, for instance either the stack or packet of metal sheet rings or the assembly of a corresponding number of a metal sheet stack for four coils is inserted in an injection molding tool and is insert molded with plastic. In this injection molding process, a thread style profile is provided in the area surrounding the teeth. While this makes the injections mold more complex and more expensive, it provides, on the other hand, that in the winding process at a later stage of the manufacturing process, when wire is wound around the teeth, the available space is used in an optimal way. The stator insulation in the form of an injection molded plastic part also serves to keep the metal sheet packet together. The single metal sheets are superimposed in a non-conductive manner, for instance with a non-conductive insulation layer in between.
As follows from the above, providing and using single coil groups, for instance coil groups of four coils, provides advantages in the process of manufacturing not only as far as punching is concerned but also in the process of insert molding since not the entire ring needs to be insert molded but just the respective coil groups of four coils which makes it possible to use a considerably smaller and therefore less expensive injection mold.
However, according to the prior art as known from DE 1 99 83 946 B4, two different kinds of coil groups of four coils are needed and, accordingly, also two different kinds of injection molding forms need to be provided. The prior art lacks any suggestion, solution or means to implement a single coil group solution. Thus there is a need in the art for means and methods of powering or otherwise enabling a single coil group solution.
In other words, the prior art, in particularly DE 1 99 83 946 B4, assigned to EPS Energy and Propulsion Systems, Inc. requires two different sets of coil groups for each phase. Referring to prior art FIG. 3, exemplifying the EPS Energy and Propulsion Systems prior art cited herein, four groups of coils are illustrated. These four groups represent phase U of a three phase system. The first coil group is marked 1 to 4, the second coil group 13 to 16, the third coil group 25 to 28 and the fourth coil group 37 to 40. Each of the four coil groups has four coils, but other configurations are contemplated. The first group starts out with a first coil (1 ccw) having a counter clockwise winding. The next three coils, 2 to 4, alternate the direction of winding. As mentioned above, the tooling, fabrication and/or injection molding for a group of coils is unduly expensive and labor intensive. Unfortunately, for the next coil group in the same phase, a completely different coil group is required. The second group starts out at 13 cw with a clockwise winding. The remaining coils of the second group, 14 to 16, alternate winding direction. A quick review and comparison of 1 ccw, 13 cw, 25 ccw and 37 cw illustrates the prior art shortfall of two different coil groups for the same phase. Thus, there is ample room in the art for improvement.